1. Field of the Invention
The present invention relates to a method of preparing nitrile rubber compositions having high modulus. The compositions according to the present invention have not only excellent oil resistance and high-temperature performances, but also improved physical properties and abrasion resistance as compared with conventional nitrile rubber compositions.
2. Description of the Prior Art
Recently, it has been demanded to develop rubbers having high modulus, excellent physical properties and abrasion resistance, particularly rubbers exhibiting excellent durability in oil or water or under severe abrading conditions.
In sulfur-curable rubber compositions, it is generally known that the crosslinking density is increased by high loading of carbon black or by increasing the amount of sulfur added in order to produce a rubber having high modulus. In this case, however, the physical properties are apt to lower and particularly, the lowering of elongation at break and flexing fatigue resistance becomes conspicuous.
Further, polyurethane rubbers have been known as an elastomer having high modulus. However, the polyurethane rubbers have various drawbacks, for example, the cost of the raw materials is high, the curing time is long, the physical properties at a low temperature are poor, the heat resistance is poor, and the abrasion resistance, flexing fatigue resistance and heat evolution characteristics are insufficient, so that the products produced from the polyurethane rubbers are not satisfactory as an industrial usage and the development of the other suitable materials has been demanded.
On the other hand, there have been known carboxylated nitrile rubber compositions, in which carboxyl group is directly added to a main chain of nitrile rubber, as a rubber composition having an oil resistance and a relatively high modulus. However, these compositions are apt to be scorched under usual compounding conditions. In order to prevent the scorching, there have been proposed various methods, one of which is the use of zinc oxide encapsulated with zinc sulfide or zinc phosphate in the curing (C. L. Bryant, Januarl of IRI, 1970, p. 202) and the other of which is the use of zinc peroxide (R. J. Weir and W. D. Gunter, European Rubber Journal, 1978, p. 20). In these methods, however, there are such drawbacks that the cost becomes expensive and the properties of the resulting composition are apt to lower.
Moreover, a method of producing elastic rubber compositions, wherein carboxy-containing polymer obtained by emulsion polymerization of diene monomer with methacrylic acid or by reaction of diene polymer with maleic anhydride (in this case, carboxyl group is bonded to the polymer main chain) is cured with a polyvalent metallic oxide such as zinc oxide or the like, has been described in U.S. Pat. No. 2,724,707. In this elastic rubber composition, however, the polymer main chains are connected to each other by ionic bonding through carboxyl group, so that the modulus and physical properties are not yet sufficient as an industrial usage.